This application relates to hard disc drives and more particularly to an apparatus and method for detecting, during the course of disc drive manufacture, a head possessing the undesirable characteristic of baseline instability.
In a disc drive, the recorded data is detected by a read/write head when the field of a recorded signal is brought in close proximity to the head. An inductive head detects a change in magnetic flux and generates a current. Detection circuitry responds to the induced current, identifying it as indicative of stored data. Alternatively, when a magneto-resistive head reads a magnetic field, it alters its own resistance. The detection circuitry detects the change in head resistance by continually running a current through the head, and identifying changes in voltage across the head. Regardless of how detection is to be accomplished, it is essential that the head""s response to encountering a localized magnetic field of a recorded signal be predictable and repeatable. Variance in head response would frustrate the detection circuitry""s ability to recognize data and result in errors during read operations.
Magneto-resistive heads possess a particular failure mechanism that is inconsistent with the goal of consistent response to magnetic field immersion. A magneto-resistive head is composed of a film, which changes its own resistance in the presence of a magnetic field. The resistance of such a head is determined by the magnetic polarity of the film. Occasionally, a magneto-resistive head will suddenly change its resistance-flux profile. This failure mechanism is the result of a given region within the magneto-resistive head suddenly re-orienting its magnetic moment in a direction differing from the orientation previously prevailing. The characteristic result of this failure mechanism is two-fold: the voltage peak resulting from the head having encountered a magnetic field is elevated, as is the baseline voltage across the head (the baseline voltage is the voltage across the head in the absence of any magnetic field). Accordingly, this failure mechanism is typically observed during a read operation. A magneto-resistive head exhibiting this failure mechanism is said to possess baseline instability, and produces errors during the reading process.
This phenomenon of baseline instability may also be observed after a write operation. During a write operation, the inductive portion (writing portion) of a magneto-resistive head generates a magnetic field, which is to be retained locally on the recording media. The magnetic field generated during a write operation is strong enough to cause the reading portion of the magneto-resistive head to alter its resistance in a subsequent read mode, thereby producing the aforementioned characteristic effects of baseline instability in susceptible heads.
The method and apparatus in accordance with the present invention solves the aforementioned problem and other problems by defining a threshold based upon the signal average amplitude of a signal emanating from a target magneto-resistive head, and counting the number of times which the signal emanating from the magneto-resistive head transcends the threshold. The method commences by capturing a signal average amplitude of a signal emanating from a magneto-resistive head which is being tested. The signal average amplitude is used to set a threshold, the transcendence of which is indicative of a baseline instability event. Finally, each baseline instability event is counted.
This method allows a population of magneto-resistive heads to be screened, so that heads possessing the instability failure mechanism can be excluded from the manufacture of drives. This method is also useful in characterizing the manufacturing process that is producing the heads.
The apparatus includes an envelope detector, which is used to capture the signal average amplitude of a signal emanating from a target magneto-resistive head. A comparator unit is operably connected to the envelope detector. The comparator unit uses a threshold based upon the signal average amplitude, and sets a flag in the event that the signal emanating from the magneto-resistive head exceeds the threshold. Finally, a counter unit is operably connected to the comparator unit. The counter unit increments its count total each time the flag indicator or mark is set. The counting unit may employ some form of timing control, meaning that the counting unit will only increment its count total if the flag is set at the same time a clock signal triggers the counter unit to test the flag for the purpose of deciding whether or not to increment.
These and various other features as well as advantages of the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings.